PTL 1 describes one example of a noise estimating system and a noise attenuation system that performs noise reduction using noise estimated by the noise estimating system. FIG. 8 is a block diagram illustrating a configuration of the noise estimating system described in PTL 1. As illustrated in FIG. 8, the noise estimating system described in PTL 1 includes a first noise estimating unit 611, a first noise attenuation unit 621, a sound pattern storage unit 631, a second noise attenuation unit 622, and a second noise estimating unit 612. The noise estimating system having such a configuration operates as follows.
The first noise estimating unit 611 estimates noise contained in an input signal and the first noise attenuation unit 621 reduces, from the input signal, a first noise estimated to obtain a first noise-attenuated signal. Using the first noise-attenuated signal and a sound pattern stored on the sound pattern storage unit 631, the second noise attenuation unit 622 obtains a second noise-attenuated signal. Using the second noise-attenuated signal, the second noise estimating unit 612 determines a second noise.
PTL 2 describes a technique for enhancing noise resistance in an environment where in addition to stationary noise, non-stationary noise such as sounds of a CD player or a radio exists.